Method and device including reworkable alpha particle barrier and corrosion barrier

ABSTRACT

A method and device comprising an easily reworkable alpha particle barrier is provided. The easily reworkable alpha particle barrier is applied in the space between the surface of the chip and the surface of the substrate, and reduces soft error rate (SER). Further, the easily reworkable alpha particle barrier material is chosen from the group of an organic material, a hydrocarbon, more specifically a polyalphaolefin (PAO) oil, and a polymer or filled polymer; wherein the polyalphaolefin oil has a viscosity below 1000 cSt (at 100° C.). The easily reworkable alpha particle barrier material can be used with multichip modules (MCM&#39;s) allowing easy device rework of one or more dies without affecting other dies on the same substrate.

FIELD OF THE INVENTION

The invention relates generally to an easily reworkable alpha particlebarrier and C4 corrosion barrier. In some aspects the invention alsorelates to a method of minimizing soft error rate (SER) of electronicdevices while allowing simple chip rework.

BACKGROUND OF THE INVENTION

Electronic components utilizing integrated circuit chips are used in anumber of applications. Controlled Collapse Chip Connection (C4 or “flipchip”) is an interconnect technology developed by IBM that provides alarge chip to package I/O capability. Typically, one or more integratedcircuit chips are mounted above a single or multiple layer substrate andpads on the chip are electrically connected to corresponding pads on thesubstrate by a plurality of electrical connections such as solder bumps.

The die-attached flip chip package typically includes a gap or spacebetween the integrated circuit chip and the substrate resulting from theoverall height of the solder bump connection between the chip and thesubstrate or adjacent carrier. The substrate material and/or theinterconnect solder can emit alpha particles. Alpha particle emissionsresult from the radioactive decay of impurity elements such as polonium,thorium and uranium in the substrate or the interconnect solder. Thealpha particles can cause changes in both logic and memory functions ofthe device in close proximity to the alpha particle source. Such adisruption of the normal operation of the semiconductor devices iscommonly referred to as a soft error upset (SEU) and the overallcontribution to the performance of the device is described as the softerror rate (SER).

The C4 joints are susceptible to cyclic fatigue cracking due to theexpansion mis-match between the chip and the underlying substrate, orcarrier. In order to improve the reliability of the solder joints undercyclic field operating conditions of the package (on/off), the gap isoften filled with an underfill material. The underfill material is oftena particle-filled epoxy compound that enhances the fatigue life of theC4. The underfill material also helps to absorb the alpha particleradiation. The fillers in the epoxy underfills should be of high purityto assure that the underfill itself is not a source of alpha particles.

There are, however, packages that do not contain the traditional epoxyunderfill and hence are susceptible to SER. These packages are typicallylarge multi chip modules (MCMs) that require chip reworkabilty forupgrades or defective chip replacement. Removing the hardened, or curedunderfill (such as the conventional epoxy underfills) for chip rework iscomplicated, if not impractical. There are also other packages which donot contain the epoxy underfill such as those that do not suffer fromthe expansion mis-match between the chip and the carrier, or those thatare not subjected to large cyclic temperature excursions in fieldoperation.

Earlier MCM packages contained thin film technology, a thin film ofmetal and/or a polymer such as a polyimide on the substrate, whichprovided the unintended benefit of absorbing the alpha radiation fromthe substrate. However, thin film technology tends to be very expensive.As described earlier, the use of hardened, or cured epoxy underfillmakes device rework extremely difficult and economically unfeasible.Therefore, there is a need to provide an effective alpha particlebarrier that is easily reworkable and cost efficient, and whichminimizes or eliminates SER.

SUMMARY OF THE INVENTION

In a first aspect, the invention provides a method of protectingelectronic circuits. The method comprises providing at least oneintegrated circuit (IC) chip electrically connected to a correspondingpad on a substrate. The method further includes providing an easilyreworkable alpha blocking material in a gap between the IC chip and thesubstrate on at least one side of the gap, an IC chip side or asubstrate side.

In a second aspect, the invention provides a method for making an easilyreworkable integrated circuit chip electronic device comprisingproviding a flip chip assembly comprising at least one chip electricallyconnected to corresponding pads on a substrate by a plurality ofconnections between the at least one chip and the substrate. There is agap between the surface of the flip chip and the surface of thesubstrate. The method further includes partially or fully filling thegap with an easily reworkable alpha blocking material selected from oneor more of an organic material, a hydrocarbon oil or an oil-fillermixture/suspension, and a polymer or filled polymer by dispensing thematerial on at least one side of the gap between the chip and thesubstrate.

A third aspect of the invention is a flip chip device which comprises analpha particle barrier material selected from the group of an organicmaterial, a PAO oil, a polymer, and a filled polymer. The alpha particlebarrier material is easily reworkable and has a viscosity less than 1000cSt at 100° C.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1-2 depict a device in accordance with the invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention relates to a method and/or device comprising an easilyreworkable alpha particle barrier layer which can also act as a C4corrosion barrier. More particularly, the invention provides a deviceand method which includes an effective alpha particle barrier layer thatis easily reworkable and which minimizes or eliminates SER. Theinvention is particularly advantageous when used with multichip modules(MCM's) as it allows easy device rework of one or more dies withoutaffecting other dies on the same substrate. The alpha particle barrierof the invention additionally acts as a corrosion barrier, effectivelykeeping moisture away from the package interconnect components.

Referring to FIG. 1, a cross-section is shown of an electronic component10 comprising an integrated circuit chip 11 and an interconnectingsubstrate 15. Integrated circuit chip 11 is shown having conductive pads12 electrically connected to conductive pads 14 by solder connections 13such as solder bumps, in a method known as C4 or flip chip packaging.The lower surface of substrate 15 may contain connectors (not shown),for connection of the substrate 15 to another electronic device such asa circuit board.

Electronic component 10 is shown in FIG. 1 as having an easilyreworkable alpha particle barrier layer 16 between pad containingsurface 17 of integrated circuit chip 11 and pad containing surface 18of substrate 15. The peripheral solder connections 13 and pads 12 and 14are shown as encapsulated by the easily reworkable alpha particlebarrier layer 16. In FIG. 2, the easily reworkable alpha particlebarrier layer 16 is shown as completely encapsulating all of the solderconnections 13.

Integrated circuit chip 11 may be any of a number of integrated circuitdevices such as a passive device or a very large scale integration(VLSI) or ultra large scale integration (ULSI) active device. Further,there may be at least one integrated circuit chip present on substrate15.

In a first embodiment, the reworkable alpha particle barrier materialcomprises a low molecular weight oligomer/polymer, such as a highviscosity grade hydrocarbon oil, for example, polyalphaolefin (PAO) oil.

Examples of oils suitable as an easily reworkable alpha particle barriermaterial in the first embodiment of the invention may include highviscosity grade oils, such as PAO 100 cSt (Mobil SHF 1003), 150, 300 or1000 cSt (Spectrasyn® grades from Exxon Mobil). The viscosity valuescorrespond to those measured at 100° C. These oils are not expected tointerfere with thermal interface materials (TIM) such as thermal paste,gels, pads, solder, etc. if any, present on the substrate and/or chip.Any oil, or low molecular weight oligomer/polymer is suitable for theinvention, provided that it performs the function of an alpha particlebarrier and can be easily removed with solvent cleaning or other means(such as heating) for chip rework. Furthermore, although oils arecontemplated, any organic compound which acts as an effective alphaparticle barrier and which can be easily removed with suitable solventcleaning or other means can be used in the present invention. Examplesare gels, low molecular weight thermoplastics, acrylates, coatings suchas Parylene, etc.

Ideally, the material is selected so as to withstand chip joining andoperational temperatures, yet still allow device reworking usingconventional cleaning tools and methods. More practically, the materialis selected so as to withstand package second-level attach tocards/boards, yet still allow chip rework using conventional cleaningtools and methods. In oil embodiments, the viscosity at 100° C. is below1000 cSt, preferably below 300 cSt, and in further embodiments belowabout 150 cSt.

Chip rework when using PAO oil or any suitable organic compound as thereworkable alpha particle barrier, can be performed for example, bywashing with xylene, rinsing with a cleaning solvent such as isopropanol(IPA), and then drying with nitrogen. Additional suitable cleaningsolvents include polar and non-polar fluids, protic and aprotic fluids,including N-methylpyrolidone (NMP), acetone, common aldehydes, ketones,tetrahydrofuran, hexafluoroisopropanol (HFIP), and gamma-butyrolactone.However, any conventional cleaning tool, solvent, or method can be usedto perform device reworking of the first embodiment of the presentinvention.

It has been found that the first embodiment of the invention is alsocapable of acting as a C4 corrosion barrier. For example, use of neatPAO oil or a mixture of PAO oil and suitable anti-oxidants such asIrganox® 1010 (antioxidant obtained from Ciba Specialty Chemicals)individually or in combination with other anti-oxidants can yieldincreased corrosion protection in hermetically, semi-hermetically andnon-hermetically sealed packages. Although the exact mechanism is notfully understood, it is believed that a hydrophobic layer coats thepackage components such as the solder interconnect, keeping moistureaway and thereby minimizing or preventing corrosion in semi- andnon-hermetic environments. Oil-solid filler compositions such asslurries and low-viscosity compounds (such as thermal paste/grease) canalso be used as an alpha particle barrier layer. These materials willprovide the additional benefit of better compatibility when used withthermal paste/grease type TIMs.

In a second embodiment, the reworkable alpha particle barrier layercomprises a polymer, or polymer/filler mixture or suspension. Examplesof polymers and compounds suitable as a reworkable alpha particlebarrier layers in the second embodiment are low molecular weightthermoplastics, such as polysulfone, polyisobutylene, acrylates,polystyrene, polyamides, etc. Other polymers such as reworkableengineered thermoplastics (for example Cookson Staystik® polymersavailable from Cookson Electronics), poly ethyl ether ketones (forexample OXPEKK® obtained from Oxford Performance Materials, Inc.), andpolyetherimide (for example available as a polymer or filled polymerfrom GE Micronized Polymers) may also be used. Suitable polymers willhave a glass transition temperature (T_(g)) between about −65° C. and230° C. and can be used neat or in combination with one another.

Optionally, the polymers such as those noted above may be filled with aceramic or metal powder in a mixture or suspension. Fillers aretypically used to increase the thermal conductivity and/or viscosity ofthe polymer and reduce the coefficient of thermal expansion. Fillers canalso function to block alpha particle emissions from the packagecomponents and/or change the dielectric properties of the alpha particlebarrier material. Examples of suitable fillers include metals such asaluminum, nickel, copper, indium, boron, tin, and silver optionally withinsulating polymer or oxide coatings. Additionally, if a metal powder isselected, it will ideally have electrically insulative properties or becoated to be electrically non-conducting to reduce C4-C4 shorting.Suitable coatings for the metal powder filler include an organic layer,for example thermoplastic or thermoset organics such aspolyamides/polyimides, or oxides such as barium titanate, Al₂O₃ or SiO₂.Other examples of suitable fillers include carbon, ceramic oxides suchas Al₂O₃, SiO₂, SiC, SnO₂, BN, AlN and diamond. The above noted fillerscan be used alone or in combination with each other. Although any amountof filler is contemplated by the present invention, it is preferable touse less than 90% filler by weight in a polymer/filler mixture orsuspension. In embodiments, the amount of filler, if any, is selecteddepending on the desired viscosity of the alpha particle barrier layerso as to enable full or partial gap filling between the substrate anddevice and/or retention of the layer between the substrate and dieduring operational temperatures.

Optionally, a solvent may be added to the neat polymer or polymer/fillermixture or suspension as a temporary carrier to lower the viscosity ofthe barrier material and aid in application to the substrate-device gap.Any solvent is suitable provided that the solvent does not substantiallydeleteriously affect the alpha blocking or dielectric properties of thebarrier during use and it is safe for handling and disposal. Afterapplication of the reworkable alpha particle barrier the solvent can beremoved by evaporation at room temperature or by heating at elevatedtemperatures, or by any other conventional method. Examples ofviscosity-lowering solvents include one or more organic solventsselected from the group of paraffins (such as heptane, octane), alcohols(ethyl, isopropyl), ethers, esters, amides, lactones, ketones, andaromatic hydrocarbons. Some typical examples includeN-methyl-pyrrolidone (NMP), diglyme, triglyme, xylene, amyl acetate, andacetophenone.

The polymer and optional fillers are selected so as to withstand package2^(nd) level attach to a printed circuit board/card and operationaltemperatures, yet still allow chip rework using conventional cleaningtools and methods. Chip rework when using a neat or filled polymer asthe reworkable alpha particle barrier can be performed, for example, bywashing with, for example, xylene, rinsing with a cleaning solvent suchas isopropanol (IPA), and then drying with nitrogen. Additional suitablecleaning solvents include polar and non-polar fluids, protic and aproticfluids, including N-methylpyrolidone, acetone, common aldehydes,ketones, tetrahydrofuran, HFIP and gamma-butyrolactone. It should benoted that any conventional cleaning tool, cleaning solvent, or methodcan be used to perform device reworking of the second embodiment of thepresent invention.

The physical state of easily reworkable alpha particle barrier materialof a second embodiment of the invention is ideally in the B-stage, thatis, in an intermediate stage in the polymerization process and notcompletely cured. Such a state allows easy reworking of the die withonly conventional cleaning techniques, and optionally heating. However,presence of the material in the B-stage is not required. Alternatively,the easily reworkable alpha particle barrier material can be a polymeror filled polymer. The invention contemplates the reworkable alphaparticle barrier material to be in any stage, provided that it is easilyreworked with only conventional cleaning techniques, and optionallyheating.

Generally, the reworkable alpha particle barrier material is manually,or with an auto-dispense tool, dispensed on at least on side of agap-between the chip and substrate to partially or fully fill the C4cage. Optionally, the substrate, or the material, or both may be treatedto reduce the viscosity and aid the flow of the alpha particle blockingmaterial, such as by way of heating, low frequency vibration, ultrasonicvibration, or combinations thereof. Techniques such as heatingfacilitate the flow of the material into the area between the substrateand the device (C4 cage). Ideally, the material is drawn into the C4cage by capillary action, and the material is expected to remain in theC4 cage by way of surface tension.

Additionally, containment methods to retain the reworkable alphaparticle barrier material in the gap between the chip and substrate canalso be included. For example, o-rings or meshes may be used to minimizeflow out from under the die region and/or to provide dam formation toretain thickness of the material and provide containment for adjacentcomponent isolation. The barrier material is at least substantiallycontained in the gap between the surface of the chip and the surface ofthe substrate, for example, after application at least a portion of thematerial remains between both the chip and the substrate, althoughsimultaneous contact with both the chip and substrate are not required.

EXAMPLE 1

A PAO oil alpha blocking material (SpectraSyn® Ultra 300) with aviscosity of about 300 cSt (at 100° C.) was dispensed with a syringearound the perimeter of chip(s) connected by C4 connections to asubstrate. Capillary action drew the material under the chip and aroundthe C4 connections, wherein the spacing between the die BLM (bondlimiting metallurgy) and substrate TSM (top surface metallurgy) arebetween about 70 and 100 μm.

The chip(s) were then tested to ensure proper connection to thesubstrate. The chip(s) can easily be removed if faulty connections arefound by washing the dispensed area with xylene, rinsing withisopropanol and then drying with nitrogen. Although not required, thechip and substrate and/or the oil can be heated to 60-85° C. to lowerthe material viscosity and facilitate the flow of the material into theC4 cage. The alpha barrier effectiveness of the oil was demonstrated byfirst measuring the alpha particle count off an MCM ceramic surface andcomparing it to the count from the same substrate surface coated with athin layer of oil as shown in table below. Alpha Counts (backgroundSample subtracted)/Cm²/Kh Baseline (Ceramic Substrate 1) 63.8 ± 7.8 Substrate 1 coated with PAO-100 oil 2.1 ± 5.2 Baseline (Ceramicsubstrate 2)  49 ± 7.9 Substrate 2 coated with 4.3 ± 5.4 PAO-150(SpectraSyn ® 150) oil PAO oils on Si wafer: SpectraSyn ® 150 3.4 ± 2.3SpectraSyn ® 300 2.8 ± 3.5 SpectraSyn ® 1000 4.8 ± 5.5

EXAMPLE 2

A low molecular weight thermoplastic polymer alpha blocking materialpolyisobutylene, was dispensed with a syringe around the perimeter ofchip(s) connected by C4 connections to a substrate. The chip-joinedsubstrate and the polymer were heated to about 85° C. to lower theviscosity of the polymer and improve its flow. Capillary action drew thematerial under the chip and around the C4 connections, wherein thespacing between the die BLM (bond limiting metallurgy) and substrate TSM(top surface metallurgy) are between about 70 and 100 μm.

The chip(s) were then tested to ensure proper connection to thesubstrate. The chip(s) can easily be removed if faulty connections arefound by washing the dispensed area with xylene, rinsing withisopropanol and then drying with nitrogen.

While the invention has been described with reference to exemplaryembodiments, it is understood that the words, which have been usedherein, are words of description and illustration, rather than words oflimitation. Changes may be made, within the purview of the appendedclaims, without departing from the scope and spirit of the presentinvention in its aspects. Thus, although the invention has beendescribed herein with reference to particular materials and embodiments,the invention is not intended to be limited to the particulars disclosedherein; rather, the invention extends to all functionally equivalentstructures, methods and uses, such as are within the scope of theappended claims.

1. A method of protecting electronic circuits comprising: providing atleast one integrated circuit (IC) chip electrically connected to acorresponding pad on a substrate; and providing an easily reworkablealpha blocking material in a gap between the IC chip and the substrateon at least one side of the gap, an IC chip side or a substrate side. 2.The method of claim 1, wherein the easily reworkable alpha blockingmaterial is selected from one or more of a hydrocarbon oil, and athermoplastic polymer.
 3. The method of claim 1 wherein the easilyreworkable alpha blocking material comprises a suspension and/or amixture of oil and inorganic filler, or a filled-polymer.
 4. The methodof claim 1, wherein the easily reworkable alpha blocking materialfurther includes a viscosity-lowering solvent selected from at least oneof paraffins, alcohols, ethers, esters, amides, lactones, ketones,sulfones, aromatic hydrocarbons, N-methyl-pyrrolidone (NMP), diglyme,triglyme, xylene, amyl acetate, and acetophenone.
 5. The method of claim1, wherein the easily reworkable alpha blocking material is easilyremoved by washing with a cleaning solvent and optionally dried withnitrogen.
 6. The method of claim 1, wherein the easily reworkable alphablocking material is drawn into a gap between the surface of the IC chipand a facing surface of the substrate by one or more of capillaryaction, heating, and vibration, and wherein the easily reworkable alphablocking material is at least substantially contained in the gap betweena surface of the IC chip and a facing surface of the substrate by atleast one of o-rings, meshes, and surface tension.
 7. The method ofclaim 2, wherein the hydrocarbon oil comprises a polyalpha olefin. 8.The method of claim 2, wherein the hydrocarbon oil has a viscosity below1000 cSt at 100° C.
 9. The method of claim 2, wherein the thermoplasticpolymer comprises a polymer selected from at least one ofpolyisobutylene, polystyrene, acrylates, poly ethyl ether ketones,polyetherimide, polyamides, and engineered thermoplastics, wherein thepolymer has a glass transition temperature (T_(g)) between about −65° C.and 230° C.
 10. The method of claim 3, wherein the filled-polymer,suspension and/or mixture comprises a metal powder filler selected fromat least one of carbon, Al₂O₃, SiO₂, SnO₂, BN, AlN, SiC diamond,aluminum, nickel, copper, indium, boron, tin, and silver, wherein saidmetal filler can optionally be coated with an organic layer, polymides,barium titanate, Al₂O₃ or SiO₂.
 11. A method for making an easilyreworkable integrated circuit chip electronic device comprising:providing a flip chip assembly comprising at least one chip electricallyconnected to corresponding pad on a substrate by a plurality ofconnections between the at least one chip and the substrate, there beinga gap between the surface of the flip chip and the surface of thesubstrate; and partially or fully filling the gap with an easilyreworkable alpha blocking material selected from one or more of anorganic material, a hydrocarbon oil or an oil-filler mixture/suspensionand a polymer or filled-polymer by dispensing the material on at leastone side of the gap between the chip and the substrate.
 12. The methodof claim 11, wherein the easily reworkable alpha blocking material has aviscosity below 1000 cSt at 100° C.
 13. The method of claim 11, whereinthe easily reworkable alpha blocking material is at least substantiallycontained in a opening between the surface of the chip and the surfaceof the substrate by o-rings or meshes.
 14. The method of claim 11,wherein the easily reworkable alpha blocking material is drawn into thegap between the surface of the chip and the surface of the substrate bya method selected from at least one of capillary action, heating, andvibration; and wherein the easily reworkable material remains in theopening between the surface of the chip and the surface of the substrateby surface tension.
 15. The method of claim 11, wherein the polymer orfilled-polymer comprises at least one polymer selected from the groupconsisting of poly ethyl ether ketones, polyetherimide, and engineeredthermoplastics, wherein the polymer or filled-polymer has a glasstransition temperature (T_(g)) between about −65° C. and 230° C.
 16. Themethod of claim 11, wherein the filled-polymer or mixture/suspensioncomprises a metal powder filler selected from at least one of carbon,Al₂O₃, SiO₂, SnO₂, BN, AlN, SiC, diamond, aluminum, nickel, copper,indium, boron, tin, and silver wherein said filler can optionally becoated with polymides, barium titanate, Al₂O₃ or SiO₂.
 17. The method ofclaim 11, wherein the easily reworkable alpha blocking material furtherincludes a viscosity lowering solvent selected from at least one ofparaffins, alcohols, ethers, esters, amides, lactones, ketones,sulfones, aromatic hydrocarbons, N-methyl-pyrrolidone (NMP), diglyme,triglyme, xylene, amyl acetate, and acetophenone.
 18. A flip chip devicewhich comprises an alpha particle barrier material selected from anorganic material, a PAO oil, a polymer, and a filled-polymer; andwherein the alpha particle barrier material is easily reworkable and hasa viscosity less than 1000 cSt at 100° C.
 19. The flip chip device ofclaim 18, wherein the polymer or filled-polymer comprises a polymerselected from at least one of poly ethyl ether ketones, polyetherimide,and engineered thermoplastics, wherein the polymer or filled-polymer hasa glass transition temperature (T_(g)) between about −65° C. and 230° C.20. The flip chip device of claim 18, wherein the filled-polymercomprises a metal powder filler selected from at least one of carbon,SiO₂, SnO₂, BN, AlN, SiC, diamond, aluminum, nickel, copper, indium,boron, and tin, wherein said filler can optionally be coated withpolymides, barium titanate, Al₂O₃ or SiO₂.